The area targeted for this study is the region bounded to the west by the Cascades, to the north by the non-extending accreted terrains of the Blue Mountains, and to the east and south by Precambrian North America (Fig. 1). Central Eastern Oregon (CEO) is among the most volcanically active regions in the western United States and incorporate the northern terminus of the Basin and Range Province in this area (Fig. 1). Both the volcanic and tectonic activity of CEO are a component of North America's response to the changing nature of interaction between North America and the various oceanic plates to the west (e.g. Atwater , 1970; Christiansen and Lipman , 1972; Cross and Pilger , 1978; Christiansen and McKee , 1978; Severinghaus and Atwater , 1990; Christiansen et al. , 2002) as well as contribution from the Yellowstone hotspot (e.g. Suppe et al ., 1975; Geist and Richards , 1993; Smith and Braile , 1994; Camp , 1995; Humphreys et al. , 2000).

Figure 1: Volcanic [after Smith and Luedke, 1984] and tectonic [after Streck et al., 1999] elements of the Pacific Northwest. Left panel shows post 17 Ma volcanic deposits to illustrate the tremendous volcanic activity east of the Cascades in the northern reaches of the Basin and Range (outlined by thick grey lines on right panel). The right panel shows only sub-5 Ma volcanic fields to illustrate the continuing activity in the Cascades and along both the High Lava Plains (HLP – brown field) and the Eastern Snake River Plain (ESRP). Short curves along the ESRP and HLP are isochrons (ages in Ma) for the migrating silicic volcanism along each volcanic trace. Flood basalt activity was fed from dike systems in the Northern Nevada Rift (NNR), Steens Mtn. (SM), the Western Snake River Plain (WSRP) and the Chief Joseph (CJ) and Cornucopia (C) dike swarms of the Columbia River basalts. With the exception of the Cornucopia swarm, these dikes occur near the western border of Precambrian North America as defined by the 0.706 line (large dot-dash line). Dotted lines show NW trending fault systems: Olympic-Wallowa Lineament (OWL), Vale (V), Brothers (B), Eugene-Denio (ED) and McLoughlin (Mc). Additional features shown include Newberry Volcano (NB), the Owyhee Plateau (OP), Juan de Fuca Plate, and San Andreas Fault (SAF).

In the Pacific Northwest, the western margin of Precambrian North America is most clearly resolved along the western Idaho shear zone, a subvertical mylonitic structural boundary coincident with an abrupt change in the initial 87 Sr/ 86 Sr of Mesozoic and Cenozoic magmatic rocks (Fig. 1). Initial 87 Sr/ 86 Sr increases rapidly from < 0.706 in the west to > 0.706 in the east across the western Idaho shear zone (e.g. Armstrong et al ., 1977; Manduca et al ., 1992). Elsewhere, the location of the Proterozoic cratonal margin is more enigmatic, but can be inferred from the extension of the "0.706 line" to the north and south. Continental material west of the cratonal margin in NE Oregon and westernmost Idaho consists of Paleozoic-Mesozoic oceanic volcanic arcs, accretionary prism complexes and associated basinal successions accreted to western North America during Middle to Late Jurassic arc-continent collision. Accretion of these terranes and related mountain building, magmatism, and basin evolution are recorded throughout the western Cordillera (e.g. Dickinson , 1979; Coney et al ., 1980; Oldow , 1984; Vallier , 1995; Bjerrum and Dorsey , 1995). At about 45 Ma, subduction jumped west to its current position and established the Cascade arc. Concurrent changes in the plate boundary configuration, relative velocity structure, and/or associated slab geometry profoundly affected the tectonic activity on the continental margin and are interpreted to be responsible for the mid-Tertiary "ignimbrite flare-up" and associated core-complex extension throughout western North America ( Lipman et al. , 1972; Christiansen and Lipman , 1972; Coney and Harms , 1984; Humphreys , 1995).

Figure 2: Looking north along the Steens Mountain fault scarp that exposes a 1 km thick section of Steens basalts along with several of the feeder dikes for this flood basalt volcano.

Beginning at ~17 Ma, massive emplacement of dikes and eruption of flood basalts occurred along a ~700 km-long north-south line following the western margin of Precambrian North America in volcanic centers that include the Northern Nevada Rift ( Zoback et al. , 1994), Steens Mountain (Fig. 2; Carlson and Hart , 1987) and the dike swarms of the Columbia River Basalt Group (CRB) in southeastern Washington (e.g. Hooper , 1997). By far the largest volumes of basalt were erupted through the northern portions of this rift including the 175,000 km 3 of the Columbia River Basalts ( Tolan et al. , 1989) and the estimated 65,000 km 3 of Steens-like basalts ( Carlson and Hart , 1987). The sudden and widespread origin of this event remains controversial and highlights the need to understand whether this event was driven by local/passive (back-arc rifting) versus external/active (plume) induced forces. Most of these large volume basalts are relatively homogeneous in composition, and quite highly differentiated (Fig. 3), implying the existence of huge staging magma chambers in the deep crust, or at the Moho ( Cox , 1980).

Figure 3: MgO vrs. Age and 87 Sr/ 86 Sr vrs Longitude for Cenozoic basalts from High Lava Plains. Steens basalts (filled symbols) tend towards low MgO whereas HAOT (open symbols on left panel) have more primitive, higher MgO compositions. HAOT (filled symbols on right panel) also display the strong regional variation in Sr isotopic composition shown by other magmas from eastern Oregon and clearly define the 0.706 line at ~117.5 o W. Data from [Hart et al., 1984; Carlson and Hart, 1987].

The large-volume, highly-differentiated, basaltic eruptions that typify the Steens-Columbia River basalts decreased dramatically by 14 Ma and ceased entirely by 12 Ma to be replaced beginning at 10.5 Ma ( Carlson and Hart , 1987) by a previously absent high-Al olivine tholeiite (HAOT; Hart , 1985) that shares many compositional similarities to mid-ocean ridge basalts (Fig. 3). HAOT was erupted from small, isolated, shields throughout CEO ( Hart et al. , 1984), along the Snake River Plain to Yellowstone ( Leeman , 1982), and at various centers in the Cascades ( McKee et al. , 1983; Bacon et al. , 1997). Experimental petrological investigations of HAOT indicate a very shallow source depth of 1.1 GPa ( Bartels et al. , 1991) showing that mantle melting has extended nearly to the base of the crust for the last 10.5 Myr. Recent low-resolution teleseismic studies of this area, however, reveal a low velocity region near 400 km depth that appears to correlate with the distribution of these primitive basalts at the surface [ Song et al. , 2003]. Given the shallow equilibration depths inferred for the HAOT, this correlation with low velocities at the top of the transition zone mandates further investigation to understand its significance.

Near the Oregon-Nevada border, the circa 17 Ma burst of flood basalt volcanism was accompanied by large volume silicic activity, such as that at McDermitt ( Conrad , 1984; Rytuba and McKee, 1984) and around the Owyhee Plateau ( Ekren et al. , 1982). Based on its in-line projection from the hotspot track along the eastern Snake River Plain, the McDermitt area often is identified as the impact point of the Yellowstone plume ( Suppe et al. , 1975; Hooper , 1997). From 17 to about 14 Ma, bimodal basalt-rhyolite volcanism was widely scattered across SE Oregon and northern Nevada (Fig. 1). By about 12 Ma, basaltic volcanism contracted towards the central part of the rift in the area that now forms the western margin and interior portions of the Owyhee Plateau ( Hart et al. , 1984; Russell et al. , 1988; Shoemaker and Hart , 2002). While surrounded by both numerous large silicic complexes and eruptive centers for the Steens-like basalts, the Owyhee Plateau contains neither (Fig. 4). Instead, volcanism in the Owyhee Plateau is dominated by younger small volume basaltic centers erupting a range of compositions from HAOT, Fe and Ti-rich basalts typical of the Snake River Plain, and rare nepheline-normative basalt ( Hart , 1985; Shoemaker and Hart , 2002). Physiographically, the Owyhee Plateau today is characterized by a relative lack of large-displacement normal faults, in contrast to surrounding regions where extension was occurring prior to and concurrent with flood basalt volcanism. Uplifted Cretaceous intrusive rocks of the Owyhee Mountains flank the eastern margin of the Owyhee Plateau and separate it from the northwest trending Western Snake River Plain - a large graben structure formed in the crust of Precambrian North America that trends roughly 90 o to the main axis of the Snake River Plain.

Figure 4: Digital elevation model for the Owyhee Plateau (dotted outline). Solid outlined areas are major silicic eruptive centers including the McDermitt (MD), SC (Santa Rosa – Calico), Juniper Mountain (JM), Silver City (SI) and Lake Owyhee (LO) caldera complexes. Also shown are major tectonic features including Steens Mtn. (SM), Northern Nevada Rift (NNR), Midas Trough (MT), Santa Rosa Mountains (SR), Bull Run – Tuscarora Mtns (BR-T), Owyhee Mtns. (OM), Oregon-Idaho Graben (OIG), and western Snake River Plain (WSRP). Red circles with blue outline are major Steens basalt eruptive loci. Red triangles are Miocene to Recent basalt eruptive centers.

After 12 Ma, both the basaltic and rhyolitic volcanism organized into 2 migrating hotspot tracks, one moving at plate speed and plate direction to the northeast up the Snake River Plain towards Yellowstone ( Pierce and Morgan , 1992) and the other moving at a similar rate to the northwest across CEO to reach Newberry Volcano today ( MacLeod et al. , 1975; Jordan et al. , 2004) forming the topographically subdued region known as the High Lava Plains (HLP: Fig. 5). After passage of the silicic volcanic "front", volcanism persists along both tracks, erupting primarily basalt to the present day along most of the High Lava Plains and Snake River Plain with concentration of Quaternary activity at both ends of both trends - Yellowstone, Newberry, and the area surrounding the Owyhee Plateau in the Jordan Valley volcanic field. At the latitude of Newberry and the Three Sisters, the Cascades arc has unusually abundant Quaternary mafic volcanic rocks (Fig. 1), owing to youthful intra-arc rifting and has a fairly thin (35 km) and mafic crust ( Sherrod and Smith , 1990; Trehu et al. , 1994; Conrey et al. , 2000). Rhyolites are also more common in the Newberry-Sisters area (Fig. 5) compared to elsewhere in the Cascades. Newberry itself is the most voluminous Quaternary volcano in Oregon. Newberry has erupted extensively during the Quaternary and most of the surface lavas are basaltic ( MacLeod et al. , 1995). The ages and eruptive history of the volcano have not been systematically studied and are the current focus of Dr. Julie Donnelly-Nolan, who will collaborate with the PIs in this project.

Figure 5: The "Big Obsidian Flow", a 1300-1500 year old obsidian flow erupted from, and flowing into, the summit caldera of Newberry volcano.

The High Lava Plains are characterized by thin, widespread Pliocene and Pleistocene primitive basalt flows, along with a 260 km long belt of silicic eruptive centers defining the McDermitt to Newberry age progressive volcanic lineament (Fig. 6). Structurally, the HLP are dominated by the northwest-striking normal faults of the Brothers fault zone (Fig. 1). The Brothers Fault Zone represents an intracontinental transform fault that is a wide, diffuse zone of northwest-striking faults that are mainly vertical, are at most a few kms long, and individually have throws of only a few tens of meters ( Lawrence , 1976, Walker and MacLeod , 1991). Basin and Range faults, which are tens of kilometers long, widely spaced, with up to kilometers of throw, are separated from the Blue Mountains stable block by the fault zone. In detail, however, Brothers Zone-type faults (NW-trend, short, small throw) are developed within and contemporaneous with the larger Basin and Range faults and are interpreted as the consequence of regional transtension ( Crider , 2001; Donath , 1962; Pezzopane and Weldon , 1993). Such faults are widely distributed in southeastern Oregon, but are concentrated in northwest-trending tracts identified, from south to north as the McLoughlin, Eugene-Denio, Brothers and Vale fault zones (Fig. 1; Lawrence , 1976). The HLP grades southward into the high relief northern Basin and Range Province in southern Oregon (Fig. 6). Oregon Basin and Range extension is characterized by conjugate sets of north-northeast and northwest trending normal faults and associated half grabens ( Donath , 1962; Walker and MacLeod , 1991; Pezzopane and Weldon , 1993; Crider , 2001) . The two fault populations have mutually cross-cutting relationships, which indicates that the two fault zones are, at least in part, contemporaneous. Extension in the Basin and Range is apparently limited in the north by the Brothers Fault Zone. GPS data identify a point on the central Oregon-Washington border as the rotation axis for extension that increases southward into the Basin and Range [e.g. McCaffrey et al. , 2000]. Paleomagnetic estimates across 42 o latitude suggest that eastern Oregon has been extended by about 17% during the last 15 Myr [ Wells and Heller , 1988] . Eastern Oregon, and the High Lava Plains in particular, thus mark the northern margin of Basin and Range related extension in western North America. In contrast to Snake River Plain volcanic trace, which displays a strong positive gravity anomaly probably reflecting abundant crust underplating by dense mafic magmas, the HLP have a much more subdued isostatic gravity signal (Fig. 6).

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